No abstract

Harvesting mechanical energy from footsteps or other impacts is a possibility to enable wireless autonomous sensor networks. These can be used for a highly efficient control of connected devices such as lights, security systems, air conditioning systems or other smart home applications. They can also be used for accurate location or occupancy monitoring. Converting the mechanical energy into useful electrical energy can be achieved using the piezoelectric effect offering simple harvesting setups and low deflections. The challenge facing piezoelectric transducers is the achievable amount of energy per impact in the lower mJ range and the management of such low energies. Simple setups for energy extraction such as a full wave bridge directly connected to a capacitor are problematic due to the mismatch between high impedance sources and low impedance storage elements. Efficient energy circuits for piezoelectric harvesters are commonly designed for vibration harvesters and require periodic inputs with predictable frequencies. Due to the sporadic nature of impact harvesters, such circuits are not well suited, since they reduce the available output. This paper discusses a piezoelectric energy harvester using a polyvinylidenfluoride (PVDF) film to capture human step-energy. Focus of this paper is a suitable energy management circuit. Furthermore, a simple wireless sensor node functioning as a switch is implemented and tested in order to validate the energy harvester capabilities to power practical applications. The implemented wireless switch consumes 58 µJ to transmit. The energy harvester stores about 122 µJ per step on a 220 µF capacitor, which is sufficient to cover the required energy and recharge a secondary battery over time. This self-sufficient approach enables a maintenance-free and indefinite operation of such wireless sensor nodes opening a wide field of new applications.